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Cobalt oxide (Co3O4) nanorings prepared from hexagonal β-Co(OH)2 nanosheets
Materials Research Bulletin 46 (2011) 1156–1162
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Cobalt oxide (Co3O4) nanorings prepared from hexagonal b-Co(OH)2 nanosheets Qiang Dong, Nobuhiro Kumada *, Yoshinori Yonesaki, Takahiro Takei, Nobukazu Kinomura Department of Research Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Miyamae cho-7, Kofu 400-8511, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 November 2010 Received in revised form 12 April 2011 Accepted 19 April 2011 Available online 27 April 2011
Hexagonal cobalt hydroxide (b-Co(OH)2) nanosheets over a size range from 100 nm to 1 mm were synthesized using a very simple hydrothermal route with cobalt naphthenate as the cobalt source. Additionally, hexagonal cobalt oxide (Co3O4) nanorings over a size range from 100 nm to 1 mm consisting of cubic nanocrystals were obtained via a hydrothermal method using as-prepared b-Co(OH)2 nanosheets as the precursors. A probable mechanism of formation of the hexagonal Co3O4 nanorings is proposed on the basis of time-dependent experimental results. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis B. Crystal growth D. Electrochemical properties
1. Introduction In recent years, research on the fundamental properties and practical applications of nanomaterials has attracted considerable attention [1]. In particular, the size effect of nanocrystals has been studied extensively. The shape of nanomaterials has a considerable effect on their physical properties and is important in many potential applications [2–8]. Thus, much effort has been exerted on exploring various approaches for preparing nanoscale materials with different sizes and shapes. Of these, the preparation of sheet-like or ring-like nanocrystals is a new and interesting research focus. For example, sheet-like CuO or SnO2 nanostructures, NiFe2O4 nanosheet, ZnO ring-like nanostructures, and iron oxide magnetic nanorings have been synthesized [9–13]. Cobalt hydroxide is attracting increasing attention because of its novel electric and catalytic properties and important technological applications [14–16]. For example, cobalt hydroxide enhances the electrochemical performance of nickel oxyhydroxide electrodes by enhancing the electrode conductivity and chargeability [17,18]. Because electric double-layer capacitance and pseudo-capacitance due to the redox reaction are both interfacial phenomena [19], materials with a sheet-like structure, which tend to form a layered structure that can provide large inter-sheet spacing for transferring ions rapidly and increase the electroactive material–electrolyte interface area, will improve the electrochem-
* Corresponding author. Tel.: +81 55 220 8615; fax: +81 55 254 3035. E-mail address: [email protected] (N. Kumada). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.04.016
ical performance. Several chemical methods have been used to prepare cobalt hydroxide with a sheet-like structure. Sampanthar and Zeng synthesized b-Co(OH)2 hexagonal sheets under flowing N2 with an ethylenediamine ligand [20]. By exfoliating assynthesized layered double hydroxide (LDH) in formamide, hexagonal Co(OH)2 nanosheets with an average lateral size of 3–4 mm and a average thickness of 60–80 nm were obtained [21]. Recently, Liu et al. have synthesized nanosheets of a- and bCo(OH)2 using hexamethylenetetramine as a hydrolysis agent [22]. Hou et al. also synthesized b-Co(OH)2 nanosheets by homogeneous precipitation with sodium hydroxide as the alkaline reagent in the presence of poly(vinylpyrrolidone) [23]. Despite these successes, there is a continuing need for simple, high-yield, environmentally benign methods for synthesizing b-Co(OH)2 nanosheets. Spinel cobalt oxide (Co3O4) is an important magnetic p-type semiconductor that has many applications in solid-state sensors, ceramic pigments, heterogeneous catalysts, rotatable magnets, electrochromic devices, lithium-ion batteries, and energy storages [24–27]. Furthermore, Co3O4 nanoparticles have interesting magnetic and field-emission properties [28,29]. Several methods have been used to synthesize nanoparticles, such as spray pyrolysis, chemical vapor deposition, sol–gel techniques, and forced hydrolysis [30–33]. A variety of novel shapes have been reported, such as Co3O4 hollow nanospheres [34], nanowalls [35], nanoboxes [36], nanocubes [37], nanofibers [38], nanorods [39], and nanotubes [40]. Recently, Co3O4 nanorings have been obtained by a thermal-decomposition method with b-Co(OH)2 nanosheets as the precursors [41]. To the best of our knowledge, however, there is no previous report on the synthesis of Co3O4 hexagonal
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nanorings consisting of cubic nanocrystals via a hydrothermal route. Here, we demonstrate that hexagonal b-Co(OH)2 nanosheets can be synthesized in large quantities by a facile hydrothermal synthetic method with cobalt naphthenate as the cobalt source under mild conditions. Our group successfully prepared hollow hematite (a-Fe2O3) microspheres via a template-free hydrothermal reaction, in which acetic acid was used for chemical etching [42]. Here, hexagonal cobalt oxide (Co3O4) nanorings are also obtained via a simple hydrothermal method using acetic acid and as-prepared hexagonal b-Co(OH)2 nanosheets as the precursors. The mechanism of formation of the hexagonal nanorings of Co3O4 is discussed on the basis of time-dependent experimental results. 2. Experimental 2.1. Synthesis 15 mL of 6% cobalt naphthenate ([(R(CH2)nCOO )2Co2+, where R is a cyclopentyl or cyclohexyl group], DIC Co. Ltd.) in xylene solution was added to 15 mL of LiOH solution (15 mol dm 3) where distilled water was used as solvent at room temperature. Then, the pink mixed solution was put in a 50 mL teflon-lined autoclave. The autoclave was sealed, and maintained at 160 8C for 1–40 h, and then cooled to room temperature naturally. After centrifugation, the products were obtained and washed with distilled water and absolute ethanol two times, and finally dried in air at 60 8C for 6 h. 0.12 mL acetic acid (CH3COOH) (6 mol dm 3) was added to 20 mL of as-prepared cobalt hydroxide solution (0.1 mol dm 3) where distilled water was used as solvent under vigorous stirring for 10 min. Then, the mixture was put in a 50 mL teflon-lined autoclave. The autoclave was sealed and maintained at 1608C for 1–30 h. The following experiment steps were the same as described above. 2.2. Characterization X-ray powder diffraction (XRD) was performed on a Rigaku Xray diffractometer (RINT2000 V Japan) with graphite-monochromatized CuKa radiation (l = 1.54056 A˚) at an acceleration voltage of 40 kV. The diffraction pattern over the range of 10–708 in 2u was recorded with a scanning speed of 38 min 1. Scanning electron microscopy (SEM) images were obtained with a JEOL JEM-6500F field emission scanning electron microscope. Transmission electron microscopy (TEM) images and selected area electron diffraction were obtained with a JEOL JEM-2000FXII transmission electron microscope at an acceleration voltage of 200 kV. The samples used for TEM observations were prepared by dispersing
Fig. 1. X-ray powder diffraction patterns of the b-Co(OH)2 produced at different reaction time at 160 8C. (a) 1 h, (b) 3 h, (c) 10 h, (d) 20 h.
products in ethanol under ultrasonic agitation for 10 min, then placing a drop of the dispersion onto a copper grid coated with a layer of amorphous carbon. 3. Results and discussion Fig. 1d shows a typical X-ray powder diffraction (XRD) pattern of the b-Co(OH)2 produced at 160 8C for 20 h. All the peaks in the XRD pattern can be indexed to the hexagonal cell of b-Co(OH)2, and are consistent with reported values (JCPDS 30-0443). No impurity phase was observed, indicating the high purity of the final products synthesized under these experimental conditions. SEM images (Fig. 2a) indicate that a large quantity of hexagonal bCo(OH)2 nanosheets with good uniformity was obtained using this approach. These nanosheets had a mean length of about 1 mm. In the high-magnification SEM image (Fig. 2b), the corners and edges of the b-Co(OH)2 nanosheets are clearly observed. The average thickness and edge size of these hexagonal nanosheets were about 100 and 600 nm, respectively. The time-dependent evolution of the product after hydrothermal treatment for 1, 3, 10, and 40 h at 160 8C is shown in Fig. 3. The product (sample #1) for 1 h consisted primarily of small hexagonal nanosheets, with a mean length of 100 nm and a thickness of 10 nm (Fig. 3a). The corresponding XRD pattern (Fig. 1a) revealed that they were also hexagonal-phase bCo(OH)2, but the crystallinity was poor compared with that of the product of the 3 h hydrothermal treatment (Fig. 1b), indicating that hydrothermal treatment was necessary for the fabrication of
Fig. 2. SEM images of the b-Co(OH)2 produced at 160 8C for 20 h.
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Fig. 3. SEM images of the b-Co(OH)2 produced at different reaction time at 160 8C. (a) 1 h, (b) 3 h, (c) 10 h, (d) 40 h.
hexagonal b-Co(OH)2 nanosheets with better crystallinity. After 3 h of hydrothermal treatment, the length and thickness of the hexagonal nanosheets (sample #2) were about 400 and 40 nm, respectively (Fig. 3b). When the hydrothermal treatment was extended to 10 h, the length and thickness of the hexagonal nanosheets (sample #3) increased to 600 and 80 nm, respectively (Fig. 3c). As the reaction time was prolonged to 20 h, hexagonal nanosheets (sample #4) with a mean length of 1 mm and a thickness of 100 nm formed (Fig. 2). Thus, the size of the hexagonal b-Co(OH)2 nanosheets can be controlled by the reaction time. There was no clear difference in the product morphology compared with that of the sample subjected to 40 h of hydrothermal treatment (Fig. 3d), demonstrating that the prolonged hydrothermal treatment was not necessary for preparing the hexagonal b-Co(OH)2 nanosheets. Interestingly, hexagonal Co3O4 nanorings can be obtained via a hydrothermal method with acetic acid and the as-prepared hexagonal b-Co(OH)2 nanosheets as the precursors at 160 8C for 10 h (here, precursors #1–4 correspond to samples #1–4, respectively). Fig. 4d shows the XRD patterns of the hexagonal Co3O4 nanorings prepared from precursor #4. All the peaks in the XRD patterns can be indexed to the pure phase of spinel cobalt oxide with lattice constant a = 8.084 A˚ (JCPDS 42-1467). No impurity peak was observed, indicating that b-Co(OH)2 was completely converted into the spinel structure Co3O4 at 160 8C for 10 h under hydrothermal treatment. Fig. 5a shows a typical SEM image of the hexagonal Co3O4 nanorings obtained via a hydrothermal treatment with precursor #4 at 160 8C for 10 h; the hexagonal shape and size are well maintained, and the average size of the hexagonal Co3O4 nanorings is about 1 mm. Upon careful observation, the hexagonal Co3O4 nanorings were found to be composed of many small cubic Co3O4 nanocrystals with a mean diameter of 100 nm (Fig. 5b); these join together to form the hexagonal Co3O4 nanorings. The hexagonal ring-like structures formed with Co3O4 nanocrystals were confirmed by TEM (Fig. 5c). The high-resolution TEM (HRTEM) image of the nanoring Co3O4 (Fig. 6a) exhibits clear lattice fringes with spacings of 0.47, 0.29 and 0.24 nm, which can be corresponded to
the 1 1 1, 2 2 0 and 2 2 2 planes of cubic Co3O4. The selected area electron diffraction (SAED) pattern is shown in Fig. 6b. The pattern reveals the good crystallinity, which can be indexed to the facecentered cubic phase of spinel Co3O4. Fig. 4a–c shows the XRD patterns of the hexagonal Co3O4 nanorings prepared from precursors #1–3. All the peaks in the XRD patterns can be indexed to the pure phase of spinel Co3O4. Comparison of the results shown in Figs 1 and 4 reveals that the crystallinity of the hexagonal Co3O4 nanorings improved with the crystallinity of the precursors. As shown in Fig. 7, the average size of the hexagonal Co3O4 nanorings also matched that of the corresponding precursors, consistent with the XRD results presented above. As the size of the precursors increased, the average size of the hexagonal Co3O4 nanorings increased from 100 to 400 to 600 nm, and all the hexagonal Co3O4 nanorings consisted of cubic Co3O4 nanocrystals of 15, 50, and 80 nm sizes, respectively.
Fig. 4. X-ray powder diffraction patterns of the Co3O4 produced from precursors #1–4 at 160 8C for 10 h.
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Fig. 5. SEM and TEM images of the Co3O4 produced from precursor #4 at 160 8C for 10 h.
Fig. 6. HRTEM image of the individual hexagonal Co3O4 nanoring produced from precursor #4 at 160 8C for 10 h. Inset: SAED pattern taken from the individual Co3O4 nanoring.
The SEM images and XRD patterns of the products produced from precursor #4 at various stages of the hydrothermal treatment are shown in Figs. 8 and 9, respectively. At the beginning of the reaction (0 h), as-prepared hexagonal nanosheets with a mean length and thickness of about 1 mm and 100 nm, respectively, were used as precursor #4 (Fig. 2), which was identified as b-Co(OH)2 by XRD (Fig. 9a). After reacting for 1 h, ring-like structures were observed at the edges of the nanosheets, although sheet-like structures remained (Fig. 8a). The product was primarily bCo(OH)2, although some Co3O4 phase was observed by XRD (Fig. 9b). As the reaction proceeded to 3 h, the sheet-like structures disappeared gradually and the ring-like structures became clear (Fig. 8b). When the reaction time exceeded 6 h, large-volume hexagonal nanorings became predominant in the product (Fig. 8c); the hexagonal nanorings were identified as primarily Co3O4 (Fig. 9c). By prolonging the reaction time to 10 h, hexagonal nanorings consisting of cubic Co3O4 nanocrystals with an average size of 100 nm formed (Fig. 5), and the hexagonal nanorings were identified as pure Co3O4 by XRD (Fig. 9d). For reaction times
exceeding 30 h, the shape of the nanorings was maintained (Fig. 8d). Temperature-dependent experiments were carried out using precursor #4 under otherwise same conditions. The product at 120 8C consisted of irregular nanosheets with a few holes, indicating that the transformation from nanosheets to nanorings was incomplete (Fig. 10a). When the temperature exceeded 120 8C (e.g., 140 and 160 8C), hexagonal nanorings were the main products (Figs. 10b and 5). However, as the temperature increased to 200 8C, cubic nanoparticles were obtained (Fig. 10c), suggesting that the temperature affected the morphology of the products. The concentration of the precursor #4 solution was also found to be important in the formation of nanorings. At a low concentration (0.05 mol dm 3) of precursor #4, no integral nanorings were observed (Fig. 11a). Increasing the concentration to 0.1 mol dm 3 resulted in the formation of integral nanorings (Fig. 5). Only a few nanorings (Fig. 11b) were obtained when the concentration was increased to 0.2 mol dm 3. Upon increasing the concentration to 0.4 mol dm 3, the products were mainly nano-
Fig. 7. SEM images of the Co3O4 produced from precursors #1–3 at 160 8C for 10 h.
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Fig. 8. SEM images of the Co3O4 produced from precursor #4 at different reaction time at 160 8C. (a) 1 h, (b) 3 h, (c) 6 h, (d) 30 h.
Fig. 9. X-ray powder diffraction patterns of the products produced from the precursor #4 at different reaction time at 160 8C. (a) 0 h, (b) 1 h, (c) 6 h, (d) 10 h.
particles (Fig. 11c). Our experiments suggested that the optimal concentration range is 0.1–0.2 mol dm 3. Etching or the partial dissolution of the interior of particles using acid is one approach for synthesizing unique nanostructures
[43,44]. Recently, our group has synthesized hollow a-Fe2O3 spheres by chemical etching using a weak acid (acetic acid) [42]. Hematite nanorings were prepared via a hydrothermal process in an acidic solution (NH4H2PO4) by etching hematite nanodisks [45]. In this work, hexagonal Co3O4 nanorings consisting of cubic Co3O4 nanocrystals were obtained from acetic acid and the as-prepared hexagonal b-Co(OH)2 nanosheets. On the basis of the above experimental results, the possible formation mechanism of the hexagonal Co3O4 nanorings in acid solution can be described as follows. In this reaction system, at a reaction time of 1 h, the edges of the nanosheets are oxidized and ring-like structures form. In the acid solution, a large number of the acetate ions may be adsorbed at the inner regions of hexagonal b-Co(OH)2 nanosheets under the hydrothermal condition. The included acetate ions at the inner regions will have a great tendency to ‘‘etch’’ the b-Co(OH)2 nanosheets, thus leading to subsequent dissolution at the inner regions. The inner regions of the b-Co(OH)2 hexagonal nanosheets are etched by acetic acid, resulting in the coexistence of ring-like and sheet-like structures (Fig. 8a). After hydrothermal treatment for 3 h, most of the sheet-like structures are broken and dissolved (Fig. 8b); in this process, b-Co(OH)2 reacts with acetic acid to produce Co(CH3COO)2. Then, after a sufficiently long reaction time (6 h), acetic acid etching of the hexagonal nanosheets proceeds and hexagonal nanorings form (Fig. 8c); simultaneously, some cubic
Fig. 10. SEM images of the product produced from precursor #4 at different reaction temperatures for 10 h. (a) 120 8C, (b) 140 8C, (c) 200 8C.
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Fig. 11. SEM images of the product produced with different concentrations of precursor #4 at 160 8C. (a) 0.05 mol dm
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, (b) 0.2 mol dm
3
, (c) 0.4 mol dm
3
.
Fig. 12. A schematic illustration of the formation mechanism of hexagonal Co3O4 nanorings produced from precursor #4.
nanocrystals appeared, indicating that b-Co(OH)2 was oxidized by the oxygen in the autoclave and transformed into Co3O4 [46]. The oxidation occurred at the edges of the nanosheets, while the inner regions of the nanosheets were etched simultaneously. Finally, hexagonal nanorings consisting of cubic Co3O4 nanocrystals formed (Fig. 5). A schematic illustration of the etching–oxidation formation mechanism of Co3O4 nanorings is shown in Fig. 12. 4. Conclusions We developed a simple method of synthesizing hexagonal Co3O4 nanorings under mild conditions using as-prepared bCo(OH)2 nanosheets as the precursors. The hexagonal Co3O4 nanorings consisted of cubic nanocrystals and the size of the hexagonal Co3O4 nanorings could be controlled by adjusting the size of the precursor. A possible mechanism of formation of the hexagonal Co3O4 nanorings was proposed on the basis of timedependent experiments that demonstrated that etching and oxidation were responsible for the formation of the Co3O4 nanorings. Acknowledgements This work was funded by the Sasakawa Scientific Research Grant from the Japan Science Society. We thank DIC Co. Ltd. for providing 6% cobalt naphthenate in xylene solution. Also this work was supported partially by Grant-in-Aid Scientific Research (C) (22560667) from the Ministry of Education, Science and Culture, Japan. References [1] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435–445. [2] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425–2427. [3] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-sayed, Science 272 (1996) 1924–1925. [4] H. Mattoussi, L.H. Radzilowski, B.O. Dabbousi, E.L. Thomas, M.G. Bawendi, M.F. Rubner, J. Appl. Phys. 83 (1998) 7965–7974. [5] Q. Dong, D. Wang, J.X. Yao, N. Kumada, N. Kinomura, T. Takei, Y. Yonesaki, Q. Cai, J. Ceram. Soc. Jpn. 117 (2009) 245–248.
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Cobalt oxide (Co3O4) nanorings prepared from hexagonal b-Co(OH)2 nanosheets Qiang Dong, Nobuhiro Kumada *, Yoshinori Yonesaki, Takahiro Takei, Nobukazu Kinomura Department of Research Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Miyamae cho-7, Kofu 400-8511, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 November 2010 Received in revised form 12 April 2011 Accepted 19 April 2011 Available online 27 April 2011
Hexagonal cobalt hydroxide (b-Co(OH)2) nanosheets over a size range from 100 nm to 1 mm were synthesized using a very simple hydrothermal route with cobalt naphthenate as the cobalt source. Additionally, hexagonal cobalt oxide (Co3O4) nanorings over a size range from 100 nm to 1 mm consisting of cubic nanocrystals were obtained via a hydrothermal method using as-prepared b-Co(OH)2 nanosheets as the precursors. A probable mechanism of formation of the hexagonal Co3O4 nanorings is proposed on the basis of time-dependent experimental results. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis B. Crystal growth D. Electrochemical properties
1. Introduction In recent years, research on the fundamental properties and practical applications of nanomaterials has attracted considerable attention [1]. In particular, the size effect of nanocrystals has been studied extensively. The shape of nanomaterials has a considerable effect on their physical properties and is important in many potential applications [2–8]. Thus, much effort has been exerted on exploring various approaches for preparing nanoscale materials with different sizes and shapes. Of these, the preparation of sheet-like or ring-like nanocrystals is a new and interesting research focus. For example, sheet-like CuO or SnO2 nanostructures, NiFe2O4 nanosheet, ZnO ring-like nanostructures, and iron oxide magnetic nanorings have been synthesized [9–13]. Cobalt hydroxide is attracting increasing attention because of its novel electric and catalytic properties and important technological applications [14–16]. For example, cobalt hydroxide enhances the electrochemical performance of nickel oxyhydroxide electrodes by enhancing the electrode conductivity and chargeability [17,18]. Because electric double-layer capacitance and pseudo-capacitance due to the redox reaction are both interfacial phenomena [19], materials with a sheet-like structure, which tend to form a layered structure that can provide large inter-sheet spacing for transferring ions rapidly and increase the electroactive material–electrolyte interface area, will improve the electrochem-
* Corresponding author. Tel.: +81 55 220 8615; fax: +81 55 254 3035. E-mail address: [email protected] (N. Kumada). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.04.016
ical performance. Several chemical methods have been used to prepare cobalt hydroxide with a sheet-like structure. Sampanthar and Zeng synthesized b-Co(OH)2 hexagonal sheets under flowing N2 with an ethylenediamine ligand [20]. By exfoliating assynthesized layered double hydroxide (LDH) in formamide, hexagonal Co(OH)2 nanosheets with an average lateral size of 3–4 mm and a average thickness of 60–80 nm were obtained [21]. Recently, Liu et al. have synthesized nanosheets of a- and bCo(OH)2 using hexamethylenetetramine as a hydrolysis agent [22]. Hou et al. also synthesized b-Co(OH)2 nanosheets by homogeneous precipitation with sodium hydroxide as the alkaline reagent in the presence of poly(vinylpyrrolidone) [23]. Despite these successes, there is a continuing need for simple, high-yield, environmentally benign methods for synthesizing b-Co(OH)2 nanosheets. Spinel cobalt oxide (Co3O4) is an important magnetic p-type semiconductor that has many applications in solid-state sensors, ceramic pigments, heterogeneous catalysts, rotatable magnets, electrochromic devices, lithium-ion batteries, and energy storages [24–27]. Furthermore, Co3O4 nanoparticles have interesting magnetic and field-emission properties [28,29]. Several methods have been used to synthesize nanoparticles, such as spray pyrolysis, chemical vapor deposition, sol–gel techniques, and forced hydrolysis [30–33]. A variety of novel shapes have been reported, such as Co3O4 hollow nanospheres [34], nanowalls [35], nanoboxes [36], nanocubes [37], nanofibers [38], nanorods [39], and nanotubes [40]. Recently, Co3O4 nanorings have been obtained by a thermal-decomposition method with b-Co(OH)2 nanosheets as the precursors [41]. To the best of our knowledge, however, there is no previous report on the synthesis of Co3O4 hexagonal
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nanorings consisting of cubic nanocrystals via a hydrothermal route. Here, we demonstrate that hexagonal b-Co(OH)2 nanosheets can be synthesized in large quantities by a facile hydrothermal synthetic method with cobalt naphthenate as the cobalt source under mild conditions. Our group successfully prepared hollow hematite (a-Fe2O3) microspheres via a template-free hydrothermal reaction, in which acetic acid was used for chemical etching [42]. Here, hexagonal cobalt oxide (Co3O4) nanorings are also obtained via a simple hydrothermal method using acetic acid and as-prepared hexagonal b-Co(OH)2 nanosheets as the precursors. The mechanism of formation of the hexagonal nanorings of Co3O4 is discussed on the basis of time-dependent experimental results. 2. Experimental 2.1. Synthesis 15 mL of 6% cobalt naphthenate ([(R(CH2)nCOO )2Co2+, where R is a cyclopentyl or cyclohexyl group], DIC Co. Ltd.) in xylene solution was added to 15 mL of LiOH solution (15 mol dm 3) where distilled water was used as solvent at room temperature. Then, the pink mixed solution was put in a 50 mL teflon-lined autoclave. The autoclave was sealed, and maintained at 160 8C for 1–40 h, and then cooled to room temperature naturally. After centrifugation, the products were obtained and washed with distilled water and absolute ethanol two times, and finally dried in air at 60 8C for 6 h. 0.12 mL acetic acid (CH3COOH) (6 mol dm 3) was added to 20 mL of as-prepared cobalt hydroxide solution (0.1 mol dm 3) where distilled water was used as solvent under vigorous stirring for 10 min. Then, the mixture was put in a 50 mL teflon-lined autoclave. The autoclave was sealed and maintained at 1608C for 1–30 h. The following experiment steps were the same as described above. 2.2. Characterization X-ray powder diffraction (XRD) was performed on a Rigaku Xray diffractometer (RINT2000 V Japan) with graphite-monochromatized CuKa radiation (l = 1.54056 A˚) at an acceleration voltage of 40 kV. The diffraction pattern over the range of 10–708 in 2u was recorded with a scanning speed of 38 min 1. Scanning electron microscopy (SEM) images were obtained with a JEOL JEM-6500F field emission scanning electron microscope. Transmission electron microscopy (TEM) images and selected area electron diffraction were obtained with a JEOL JEM-2000FXII transmission electron microscope at an acceleration voltage of 200 kV. The samples used for TEM observations were prepared by dispersing
Fig. 1. X-ray powder diffraction patterns of the b-Co(OH)2 produced at different reaction time at 160 8C. (a) 1 h, (b) 3 h, (c) 10 h, (d) 20 h.
products in ethanol under ultrasonic agitation for 10 min, then placing a drop of the dispersion onto a copper grid coated with a layer of amorphous carbon. 3. Results and discussion Fig. 1d shows a typical X-ray powder diffraction (XRD) pattern of the b-Co(OH)2 produced at 160 8C for 20 h. All the peaks in the XRD pattern can be indexed to the hexagonal cell of b-Co(OH)2, and are consistent with reported values (JCPDS 30-0443). No impurity phase was observed, indicating the high purity of the final products synthesized under these experimental conditions. SEM images (Fig. 2a) indicate that a large quantity of hexagonal bCo(OH)2 nanosheets with good uniformity was obtained using this approach. These nanosheets had a mean length of about 1 mm. In the high-magnification SEM image (Fig. 2b), the corners and edges of the b-Co(OH)2 nanosheets are clearly observed. The average thickness and edge size of these hexagonal nanosheets were about 100 and 600 nm, respectively. The time-dependent evolution of the product after hydrothermal treatment for 1, 3, 10, and 40 h at 160 8C is shown in Fig. 3. The product (sample #1) for 1 h consisted primarily of small hexagonal nanosheets, with a mean length of 100 nm and a thickness of 10 nm (Fig. 3a). The corresponding XRD pattern (Fig. 1a) revealed that they were also hexagonal-phase bCo(OH)2, but the crystallinity was poor compared with that of the product of the 3 h hydrothermal treatment (Fig. 1b), indicating that hydrothermal treatment was necessary for the fabrication of
Fig. 2. SEM images of the b-Co(OH)2 produced at 160 8C for 20 h.
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Fig. 3. SEM images of the b-Co(OH)2 produced at different reaction time at 160 8C. (a) 1 h, (b) 3 h, (c) 10 h, (d) 40 h.
hexagonal b-Co(OH)2 nanosheets with better crystallinity. After 3 h of hydrothermal treatment, the length and thickness of the hexagonal nanosheets (sample #2) were about 400 and 40 nm, respectively (Fig. 3b). When the hydrothermal treatment was extended to 10 h, the length and thickness of the hexagonal nanosheets (sample #3) increased to 600 and 80 nm, respectively (Fig. 3c). As the reaction time was prolonged to 20 h, hexagonal nanosheets (sample #4) with a mean length of 1 mm and a thickness of 100 nm formed (Fig. 2). Thus, the size of the hexagonal b-Co(OH)2 nanosheets can be controlled by the reaction time. There was no clear difference in the product morphology compared with that of the sample subjected to 40 h of hydrothermal treatment (Fig. 3d), demonstrating that the prolonged hydrothermal treatment was not necessary for preparing the hexagonal b-Co(OH)2 nanosheets. Interestingly, hexagonal Co3O4 nanorings can be obtained via a hydrothermal method with acetic acid and the as-prepared hexagonal b-Co(OH)2 nanosheets as the precursors at 160 8C for 10 h (here, precursors #1–4 correspond to samples #1–4, respectively). Fig. 4d shows the XRD patterns of the hexagonal Co3O4 nanorings prepared from precursor #4. All the peaks in the XRD patterns can be indexed to the pure phase of spinel cobalt oxide with lattice constant a = 8.084 A˚ (JCPDS 42-1467). No impurity peak was observed, indicating that b-Co(OH)2 was completely converted into the spinel structure Co3O4 at 160 8C for 10 h under hydrothermal treatment. Fig. 5a shows a typical SEM image of the hexagonal Co3O4 nanorings obtained via a hydrothermal treatment with precursor #4 at 160 8C for 10 h; the hexagonal shape and size are well maintained, and the average size of the hexagonal Co3O4 nanorings is about 1 mm. Upon careful observation, the hexagonal Co3O4 nanorings were found to be composed of many small cubic Co3O4 nanocrystals with a mean diameter of 100 nm (Fig. 5b); these join together to form the hexagonal Co3O4 nanorings. The hexagonal ring-like structures formed with Co3O4 nanocrystals were confirmed by TEM (Fig. 5c). The high-resolution TEM (HRTEM) image of the nanoring Co3O4 (Fig. 6a) exhibits clear lattice fringes with spacings of 0.47, 0.29 and 0.24 nm, which can be corresponded to
the 1 1 1, 2 2 0 and 2 2 2 planes of cubic Co3O4. The selected area electron diffraction (SAED) pattern is shown in Fig. 6b. The pattern reveals the good crystallinity, which can be indexed to the facecentered cubic phase of spinel Co3O4. Fig. 4a–c shows the XRD patterns of the hexagonal Co3O4 nanorings prepared from precursors #1–3. All the peaks in the XRD patterns can be indexed to the pure phase of spinel Co3O4. Comparison of the results shown in Figs 1 and 4 reveals that the crystallinity of the hexagonal Co3O4 nanorings improved with the crystallinity of the precursors. As shown in Fig. 7, the average size of the hexagonal Co3O4 nanorings also matched that of the corresponding precursors, consistent with the XRD results presented above. As the size of the precursors increased, the average size of the hexagonal Co3O4 nanorings increased from 100 to 400 to 600 nm, and all the hexagonal Co3O4 nanorings consisted of cubic Co3O4 nanocrystals of 15, 50, and 80 nm sizes, respectively.
Fig. 4. X-ray powder diffraction patterns of the Co3O4 produced from precursors #1–4 at 160 8C for 10 h.
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Fig. 5. SEM and TEM images of the Co3O4 produced from precursor #4 at 160 8C for 10 h.
Fig. 6. HRTEM image of the individual hexagonal Co3O4 nanoring produced from precursor #4 at 160 8C for 10 h. Inset: SAED pattern taken from the individual Co3O4 nanoring.
The SEM images and XRD patterns of the products produced from precursor #4 at various stages of the hydrothermal treatment are shown in Figs. 8 and 9, respectively. At the beginning of the reaction (0 h), as-prepared hexagonal nanosheets with a mean length and thickness of about 1 mm and 100 nm, respectively, were used as precursor #4 (Fig. 2), which was identified as b-Co(OH)2 by XRD (Fig. 9a). After reacting for 1 h, ring-like structures were observed at the edges of the nanosheets, although sheet-like structures remained (Fig. 8a). The product was primarily bCo(OH)2, although some Co3O4 phase was observed by XRD (Fig. 9b). As the reaction proceeded to 3 h, the sheet-like structures disappeared gradually and the ring-like structures became clear (Fig. 8b). When the reaction time exceeded 6 h, large-volume hexagonal nanorings became predominant in the product (Fig. 8c); the hexagonal nanorings were identified as primarily Co3O4 (Fig. 9c). By prolonging the reaction time to 10 h, hexagonal nanorings consisting of cubic Co3O4 nanocrystals with an average size of 100 nm formed (Fig. 5), and the hexagonal nanorings were identified as pure Co3O4 by XRD (Fig. 9d). For reaction times
exceeding 30 h, the shape of the nanorings was maintained (Fig. 8d). Temperature-dependent experiments were carried out using precursor #4 under otherwise same conditions. The product at 120 8C consisted of irregular nanosheets with a few holes, indicating that the transformation from nanosheets to nanorings was incomplete (Fig. 10a). When the temperature exceeded 120 8C (e.g., 140 and 160 8C), hexagonal nanorings were the main products (Figs. 10b and 5). However, as the temperature increased to 200 8C, cubic nanoparticles were obtained (Fig. 10c), suggesting that the temperature affected the morphology of the products. The concentration of the precursor #4 solution was also found to be important in the formation of nanorings. At a low concentration (0.05 mol dm 3) of precursor #4, no integral nanorings were observed (Fig. 11a). Increasing the concentration to 0.1 mol dm 3 resulted in the formation of integral nanorings (Fig. 5). Only a few nanorings (Fig. 11b) were obtained when the concentration was increased to 0.2 mol dm 3. Upon increasing the concentration to 0.4 mol dm 3, the products were mainly nano-
Fig. 7. SEM images of the Co3O4 produced from precursors #1–3 at 160 8C for 10 h.
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Fig. 8. SEM images of the Co3O4 produced from precursor #4 at different reaction time at 160 8C. (a) 1 h, (b) 3 h, (c) 6 h, (d) 30 h.
Fig. 9. X-ray powder diffraction patterns of the products produced from the precursor #4 at different reaction time at 160 8C. (a) 0 h, (b) 1 h, (c) 6 h, (d) 10 h.
particles (Fig. 11c). Our experiments suggested that the optimal concentration range is 0.1–0.2 mol dm 3. Etching or the partial dissolution of the interior of particles using acid is one approach for synthesizing unique nanostructures
[43,44]. Recently, our group has synthesized hollow a-Fe2O3 spheres by chemical etching using a weak acid (acetic acid) [42]. Hematite nanorings were prepared via a hydrothermal process in an acidic solution (NH4H2PO4) by etching hematite nanodisks [45]. In this work, hexagonal Co3O4 nanorings consisting of cubic Co3O4 nanocrystals were obtained from acetic acid and the as-prepared hexagonal b-Co(OH)2 nanosheets. On the basis of the above experimental results, the possible formation mechanism of the hexagonal Co3O4 nanorings in acid solution can be described as follows. In this reaction system, at a reaction time of 1 h, the edges of the nanosheets are oxidized and ring-like structures form. In the acid solution, a large number of the acetate ions may be adsorbed at the inner regions of hexagonal b-Co(OH)2 nanosheets under the hydrothermal condition. The included acetate ions at the inner regions will have a great tendency to ‘‘etch’’ the b-Co(OH)2 nanosheets, thus leading to subsequent dissolution at the inner regions. The inner regions of the b-Co(OH)2 hexagonal nanosheets are etched by acetic acid, resulting in the coexistence of ring-like and sheet-like structures (Fig. 8a). After hydrothermal treatment for 3 h, most of the sheet-like structures are broken and dissolved (Fig. 8b); in this process, b-Co(OH)2 reacts with acetic acid to produce Co(CH3COO)2. Then, after a sufficiently long reaction time (6 h), acetic acid etching of the hexagonal nanosheets proceeds and hexagonal nanorings form (Fig. 8c); simultaneously, some cubic
Fig. 10. SEM images of the product produced from precursor #4 at different reaction temperatures for 10 h. (a) 120 8C, (b) 140 8C, (c) 200 8C.
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Fig. 11. SEM images of the product produced with different concentrations of precursor #4 at 160 8C. (a) 0.05 mol dm
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, (b) 0.2 mol dm
3
, (c) 0.4 mol dm
3
.
Fig. 12. A schematic illustration of the formation mechanism of hexagonal Co3O4 nanorings produced from precursor #4.
nanocrystals appeared, indicating that b-Co(OH)2 was oxidized by the oxygen in the autoclave and transformed into Co3O4 [46]. The oxidation occurred at the edges of the nanosheets, while the inner regions of the nanosheets were etched simultaneously. Finally, hexagonal nanorings consisting of cubic Co3O4 nanocrystals formed (Fig. 5). A schematic illustration of the etching–oxidation formation mechanism of Co3O4 nanorings is shown in Fig. 12. 4. Conclusions We developed a simple method of synthesizing hexagonal Co3O4 nanorings under mild conditions using as-prepared bCo(OH)2 nanosheets as the precursors. The hexagonal Co3O4 nanorings consisted of cubic nanocrystals and the size of the hexagonal Co3O4 nanorings could be controlled by adjusting the size of the precursor. A possible mechanism of formation of the hexagonal Co3O4 nanorings was proposed on the basis of timedependent experiments that demonstrated that etching and oxidation were responsible for the formation of the Co3O4 nanorings. Acknowledgements This work was funded by the Sasakawa Scientific Research Grant from the Japan Science Society. We thank DIC Co. Ltd. for providing 6% cobalt naphthenate in xylene solution. Also this work was supported partially by Grant-in-Aid Scientific Research (C) (22560667) from the Ministry of Education, Science and Culture, Japan. References [1] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435–445. [2] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425–2427. [3] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-sayed, Science 272 (1996) 1924–1925. [4] H. Mattoussi, L.H. Radzilowski, B.O. Dabbousi, E.L. Thomas, M.G. Bawendi, M.F. Rubner, J. Appl. Phys. 83 (1998) 7965–7974. [5] Q. Dong, D. Wang, J.X. Yao, N. Kumada, N. Kinomura, T. Takei, Y. Yonesaki, Q. Cai, J. Ceram. Soc. Jpn. 117 (2009) 245–248.
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